SUMMARY ABSTRACT Immunity has a well-established role in Facioscapulohumeral Muscular Dystrophy (FSHD) muscle pathology, but animal models do not currently exist to investigate these mechanisms. Our project goal is to develop a mouse model of the FSHD innate immune response observed in FSHD muscles by combining muscle and hematopoietic stem cell (HSC) xeno-engraftment into the NSG-QUAD (NOD scid gamma, expressing human SCF, GM-CSF, IL3 and CSF1) mouse, which has significantly enhanced innate immune cell development. Our overriding hypothesis is that sporadic misexpression of the full-length double homeobox 4 (DUX4-fl) FSHD germ line transcription factor in FSHD muscles elicits an innate immunity response to low level muscle damage caused by misexpression of toxic DUX4-fl regulated germline proteins and abnormal proteins produced through downstream inhibition of nonsense mediated decay machinery, leading to an adaptive immune response that destroys immune targeted muscles and allows fat and connective tissue replacement. Our studies utilize immunohistology, blood cytokine and NanoString RNA assays to monitor innate immunity gene expression specifically elicited by FSHD muscle xenografts in mice with a humanized immune system. Once this model is established, the role of DUX4-fl in eliciting the innate immune response will be investigated by DUX4-fl knockdown using the DUX4-fl-specific morpholino, FM10, and the roles of specific cytokines in the FSHD innate immunity response will be investigated using cytokine inhibitors. As there are currently no animal models of immunity in human muscle diseases, this high risk, high reward project promises to provide a new experimental model of FSHD immunity to investigate FSHD muscle pathology and enable development of immune therapeutics.